A wireless network system and a network address assigning method are provided which are capable of reducing traffic occurring when an network address is assigned to each of communication units making up a tree-structure-type wireless network system.The wireless network system includes a radio-relay station, a radio-relay station connected to the radio-relay station and to another radio-relay station in a wireless manner. The radio-relay station preserves a plurality of network addresses to be assigned to these radio-relay stations, assigns a first network address belonging to the plurality of network addresses to one radio-relay station and an address pool making up the plurality of network addresses collectively to the other radio-relay station. The above radio-relay station preserves the address pool and assigns a second network address belonging to the address pool to a wireless terminal. The radio-relay station does not assign directly the network address to the wireless terminal.